The Cyclodextrins and Their Inclusion Complexes

1 Each glucose in a cyclodextrin has two acceptor functions -0(4)- (which, in fact, rarely accepts a hydrogen bond), -0(5)-, and three hydroxyl groups. In addition, there are water of hydration molecules which act as double donors and as single or double acceptors (see Part IV, Chap. 21). 

Cyclodextrins are macrocycles with limited flexibility. In all the crystal structure analyses of cyclodextrins, the glucose units have the D-pyranose configuration with the normal 4Ct chair conformation [556, 558, 559]. This chair form is nearly rigid, as illustrated by only small variations ( 7°) of the intrapyranose ring tor- 

In contrast, the primary 0(6) -H hydroxyl group can rotate about the exocyclic C(5)-C(6) bond. As with hexopyranoses discussed in Part II, Chapter 13, there are two preferred conformations + sc and - sc for the 0(6) - C(6) - C(5) - 0(5) torsion angle which are determined by the perio- or 1,3-diaxial or Hassel-Ottar effect [499, 501, 502, 560]. The ap conformation has never been found in the cyclodextrin 

The inclusion capability of the cyclodextrins is of special interest for the study of weak interac-j tions. Because of their central cavity, the cyclodextrins act as host , to form inclusion com-i pounds with guest molecules. The only requirement for inclusion is that the guest fits into the cyclodextrin cavity. Other factors such as hydrophobic, hydrophilic, molecular, or ionic I character are less important. Since the cyclodextrin cavity is a property of the covalently bond- ed molecule, it exists also in solution. Therefore the inclusion process can be investigated both.  

I Cyclodextrins are excellent enzyme models Catalysis and induced fit. Due to their cavities, which are able to accommodate guest (substrate) molecules, and due to the many hydroxyl groups lining this cavity, cyclodextrins can act catalytically in a variety of chemical reactions and they therefore serve as good model enzymes. Thus, benzoic acid esters are hydrolyzed in I aqueous solution by factors up to 100 times faster if cyclodextrins are added. The reaction in- j volves an acylated cyclodextrin as intermediate which is hydrolyzed in a second step of the j reaction, a mechanism reminiscent of the enzyme chymotrypsin. The catalytic efficiency can. be further enhanced if the cyclodextrins are suitably modified chemically so that a whole range of artificial enzymes have been synthesized [551-555, 556, 563, 564]. 

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J. Szejtli The Cyclodextrins and their Inclusion Complexes, pp. 74, Akademiai Kiado, Budapest (1982). 

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